Fair Value Of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market fund value	$ 312.9	$ 129.2
Money market fund maturity date (or less)	3 months	3 months
Convertible Senior Notes At 5.75%, Maturing 2021 \| Convertible notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of convertible debt	$ 454.6

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant